1. Field of the Invention
The present invention relates in general to a fluorescent endoscope apparatus for photographing as an image the autofluorescent light emitted from a living-tissue subject, representing the state of the tissues of the living-tissue subject, upon irradiation thereof by stimulating light.
2. Description of the Related Art
Researchers have long been researching fluorescent endoscope apparatuses for use in detecting the extremely faint fluorescent light emitted from a living-tissue subject irradiated by stimulating light, which can then be analyzed to distinguish the change of state in tissues accompanying various diseases.
For example, in in-vitro cancer diagnosis studies employing autofluorescence, by use of stimulating light having several specified wavelengths of light from within the wide spectrum covering the 330-450 nm range, it has been shown that it is possible to distinguish between normal and diseased tissues using this technique.
When light having such short wavelengths, from within the range spanning from the UV to the visible spectra, is employed as stimulating light, fluorescent light is emitted from the optical element forming the stimulating light projecting portion when the stimulating light is propagated therethrough. Also, the shorter the wavelength of the stimulating light, the higher the strength of the fluorescent light emitted from the optical element. Because of this, there are cases in which a non-fluorescent material such as silica, etc., which emits less fluorescent light when the stimulating light is transmitted therethrough, is used for forming the optical element forming the stimulating light projecting portion.
However, it was found that when a non-fluorescent material such as silica is used to form the optical element as a measure to reduced the strength of the fluorescent light emitted therefrom, fluorescent light containing the same wavelengths of the autofluorescent light is also emitted from the photographing portion photographing the image formed by the autofluorescent light (hereinafter also referred to as an autofluorescent image Zj) emitted by a living-tissue subject that has been irradiated by stimulating light. That is to say, the light reflected from the living-tissue subject irradiated by stimulating light enters the photographing portion, and by entry of this reflected stimulating light into the photographing portion, the constituent material of the optical element propagating the autofluorescent light contained within the photographing portion (for example, the components contained in multi-component glass, or the organic material component contained in colored fiber, etc.) have been found to emit fluorescent light upon exposure to stimulating light. Therefore, the fluorescent light emitted from this optical element is mixed, as noise, with the autofluorescent light entering the photographing portion, and as a result, the are instances in which an autofluorescent image containing much noise is photographed.
For example, there are studies in which, using a fiber-optic endoscope, in which the autofluorescent light is propagated by an image fiber, the strength of the fluorescent light emitted by the objective lens and image fiber has been found to be 0.8-0.9 times the value of the autofluorescent light emitted from normal tissue of a living-tissue subject. Furthermore, there are studies in which, irradiated by the same strength of stimulating light, the autofluorescent light emitted by a diseased tissue that should be the object of observation, which is {fraction (1/10)} of that of normal tissue, becomes buried in the fluorescent light, which becomes noise, produced by a forementioned objective lens and image fiber, and the diseased tissue has been spuriously diagnosed as a negative portion.
In addition, the normal observation range of a fluorescent endoscope apparatus is from a close point at a distance of 5 mm from the forward end of the endoscope portion to a far-point at a distance of 55 mm from the forward end of the endoscope portion. When the autofluorescent light emitted from the live-tissue subject located 55 mm from the forward end of the endoscope portion, which has been irradiated by the stimulating light Le projected from the forward end of the endoscope portion, is received at the photographing portion after being propagated thereto through the optical element, a portion of the stimulating light projected from the forward end of the endoscope portion toward the living-tissue subject located at a distance of 5 mm away from the forward end of the endoscope portion is reflected, and when this reflected stimulating light Lf is propagated within the optical element, the fluorescent light emitted by the optical element, which becomes noise, is received by the photographing portion together with aforementioned autofluorescent light, and even under the condition of the lowest possible S/N ratio enabling observation (that is, the condition of the observational limit), it is desirable that the diseased tissue and the normal tissue can be clearly distinguished.
The present invention has been developed in consideration of the circumstances described above, and it is a primary objective of the present invention to provide a fluorescent endoscope apparatus in which the fluorescent light, which becomes noise, emitted from the photographic element propagating the autofluorescent light emitted from a living-tissue subject is reduced, and by use of which it is possible to clearly distinguish between normal and diseased tissues.
The fluorescent endoscope apparatus according to the present invention comprises a stimulating light projecting means for irradiating a living-tissue subject with stimulating light, and a photographing means formed of an optical element for transferring the autofluorescent light emitted from the live-subject tissue upon irradiation thereof by stimulating light and a photographing element for photographing the autofluorescent light transferred thereto by said optical element, wherein said optical element is constructed so that the relationship of the normal-tissue fluorescent light of a strength K, which is the strength of the autofluorescent light emitted from the normal tissue of the live-tissue subject that has been transferred through said optical element and received by the photographing element, to the optical-element fluorescent light of a strength B, which is the strength of the fluorescent light emitted by said optical element when the reflected stimulating light reflected from the living tissue subject upon irradiation thereof by stimulating light is propagated through said optical element, satisfies the condition expressed by the formula: Kxe2x89xa7Bxc3x97104.
Note that normal-tissue fluorescent light of a strength K and optical-element fluorescent light of a strength B are measured values as described below.
As shown in FIG. 1A, the autofluorescent light Lj emitted by a living-tissue subject 1, of which all regions are normal tissue, upon irradiation thereof by stimulating light Le is passed through an optical element 2 formed of a stimulating light cutoff filter 2a that cuts off substantially 100 percent of the stimulating light and a focusing optical system 2b, and is received by a photographing element 3. On the other hand, the stimulating light reflected by living-tissue subject 1 upon irradiation thereof by stimulating light, reflected stimulating light Lh, is substantially 100 percent cutoff by cutoff filter 2a before entering focusing optical system 2b. At this point, the unit per area strength of autofluorescent light Lj received by photographing element 3 is strength K.
As shown in FIG. 1B, the positions of the stimulating light cutoff filter 2a and the focusing optical system 2b forming optical element 2 are switched and stimulating light cutoff filter 2a is disposed between photographing element 3 and focusing optical system 2b, and when the aforementioned living-tissue subject 1 is irradiated by stimulating light Le, the autofluorescent light Lj emitted by said living tissue subject 1 passes through optical element 2 and is received by photographing element 3. On the other hand, when the reflected stimulating light Lh reflected by said living-tissue subject upon irradiation thereof by stimulating light Le enters focusing optical system 2b, the constituent material forming focusing optical system 2b is stimulated by the reflected stimulating light Lh and emits fluorescent light Lk. Afterwards, reflected stimulating light Lh passes through focusing optical system 2b and before entering photographing element 3 is substantially 100 percent cutoff by stimulating light cutoff filter 2a, however, because the fluorescent light Lk emitted by said focusing optical system 2b contains light of the same wavelength range as that of the autofluorescent light Lj, said fluorescent light Lk passes through stimulating light cutoff filter 2a and is received by photographing element 3.
Here, if the unit per area strength of the product of the autofluorescent light Lj and the fluorescent light Lk received by photographing element 3 are designated as multiplied fluorescent light strength X, the value resulting from subtracting normal-tissue fluorescent light strength K from multiplied fluorescent light strength X (multiplied fluorescent light strength Xxe2x80x94normal-tissue fluorescent light of a strength K) becomes the optical element fluorescent light strength B, which represents the unit per area strength of the fluorescent light Lk emitted by optical element 2 and received by photographing element 3.
The optical element is provided with a stimulating light cutoff filter for selectively cutting off the stimulating light. The stimulating light cutoff filter can be provided as a panel of optical glass on which a multiple-layer dielectric film has been formed.
It is preferable that at least one of aforementioned optical element components through which the autofluorescent light is passed and which is disposed between the aforementioned a multiple-layer dielectric film and the living-tissue subject has properties satisfying the condition expressed by the following formulae:
xcexex greater than xcex80+(8/15)xc3x97(xcex80xe2x88x92xcex05) 
Where:
xcexex=the wavelength of the stimulating light
xcex80=the wavelength at which the optical element exhibits the transmittance of 80%
xcex05=the wavelength at which the optical element exhibits the transmittance of 5%
It is preferable that the stimulating light have a wavelength of 445 nm or smaller.
A GaN semiconductor laser, a mercury lamp, a xenon lamp, or a metal halide lamp can be employed as the stimulating light source.
Note that the referent of the aforementioned optical element includes all elements with the capability of passing stimulating light and reflecting stimulating light, and includes not only the lenses, prisms, etc., but also the mirror tubing and other support members, etc. used to support these components, as well as the adhesives, etc. used to join together or secure said components and support members, etc. in place.
According to the fluorescent endoscope apparatus of the present invention, upon photographing the autofluorescent light emitted by a living tissue subject upon irradiation thereof by stimulating light, because the optical element has been constructed so that the relationship between normal-tissue fluorescent light of a strength K and optical element fluorescent light strength B satisfies the condition expressed by the formula: Kxe2x89xa7Bxc3x97104, even under aforementioned observational limit condition, which is the condition among the observation conditions in which the most noise is contained, the difference between normal tissue and diseased tissue can be clearly distinguished.
For example, as shown in FIG. 2, stimulating light Le is projected from end face 40a of endoscope forward end portion 40, and the value of the far-point autofluorescent light strength Fj, which is the unit per area strength of the autofluorescent light Lj emitted by the normal tissue of a living-tissue subject located at a far-point P1 at a distance of 50 mm from end face 40a of and which has been passed through optical element 2 and received by photographing element 3, is 100 (Fj=100). On the other hand, the value of the far-point optical element fluorescent light strength Fk, which is the unit per area strength of the fluorescent light Lk emitted by the constituent material of optical element 2 upon stimulation thereof by the passing therethrough of the reflected stimulating light Lh reflected by living-tissue subject 1 located at a far-point P2 at a distance of 50 mm from end face 40a of and which has been received by photographing element 3, is less than 1/1xc3x97104 that of Fj; Fkxe2x89xa60.01. The value of the close-point optical element fluorescent light strength Ck, which is the unit per area strength of the fluorescent light Lk emitted by the constituent material of optical element 2 upon stimulation thereof by the passing therethrough of the reflected stimulating light Lh reflected by living-tissue subject 1 located at a close-point P3 at a distance of 5 mm from end face 40a of and which has been received by photographing element 3, is substantially 100 times that of the value of the far-point optical element fluorescent light strength Fk for far-point P2 (because it is {fraction (1/10)} the distance to the position of the living-tissue subject reflecting the stimulating light Le projected from end face 40a is, close-point optical element fluorescent light strength Ck is 100 times that of reflected stimulating light Lh); close-point optical element fluorescent light strength Ck is 1 or less (herein, the substantial reflectance of the stimulating light Le reflected by the living-tissue is supposed to be 1).
Accordingly, under the condition of the observational limit, in which the autofluorescent light emitted by the living-tissue subject to become the object of observation is located at a far-point is mixed with the fluorescent light, which becomes noise, emitted by the optical element when the reflected stimulating light is propagated therethrough, for cases in which said living-tissue subject is a normal tissue, the strength ratio of the autofluorescent light Lj emitted from said living-tissue subject to the fluorescent light Lk, which becomes noise, emitted from optical element 2, Ck/Fj is less than {fraction (1/100)}, and for cases in which the said living-tissue subject is diseased tissue, because the strength ratio of the autofluorescent light Lj emitted from said living-tissue subject to the fluorescent light Lk, which becomes noise, emitted from optical element 2 is less than {fraction (1/10)}, the autofluorescent light representing the living-tissue subject that is the object of observation does not become buried in the fluorescent light, which becomes noise, emitted by the optical element, and observation can be performed wherein the difference between normal and diseased tissue can be clearly distinguished.
In addition, the optical element is provided with a stimulating light cutoff filter for selectively cutting off the stimulating light, and the aforementioned stimulating light cutoff filter is provided as a panel of optical glass on which a multiple-layer dielectric film has been formed, such that the apparatus can be provided having a better configuration.
Also, if at least one of the aforementioned optical element components through which the autofluorescent light is passed and which is disposed between the aforementioned multiple-layer dielectric film and the living-tissue subject has properties satisfying the condition expressed by the formulae listed below, the fluorescent light emitted by the optical element, which becomes noise, can be more precisely controlled:
xcexex greater than xcex80+(8/15)xc3x97(xcex80xe2x88x92xcex05) 
Where:
xcexex=the wavelength of the stimulating light
xcex80=the wavelength at which the optical element exhibits the transmittance of 80%
xcex05=the wavelength at which the optical element exhibits the transmittance of 5%
If the wavelength of the stimulating light is 445 nm or smaller, the autofluorescent light emitted from a living-tissue subject upon exposure thereto can be made to be emitted more precisely.
If the light source is a GaN semiconductor laser, a mercury lamp, a xenon lamp, or a metal halide lamp, stimulating light having a wavelength of 445 nm or smaller can be more easily obtained.